1. Field of the Invention
Herein is disclosed a magnetic flux inspection probe for detecting internal and external flaws in tubular goods. The probe employs a dual-path magnetic field wherein a second magnetic field is used to constrain a first magnetic field to remain within a specified volumetric region of a specimen under inspection. By use of the disclosed system, the physical size of the magnetic probe can be substantially reduced making the device practical for use in small-diameter tubing.
2. Discussion of the Prior Art
Flaws in ferromagnetic material and in particular, in tubular goods such as, for example, pipes and well casing, are commonly detected by establishing a magnetic field in the wall thereof. For simplicity, the term "pipe" will be deemed to include all forms of tubular goods, including structural shapes having an infinite radius of curvature. Distortion of the magnetic field caused by pits, corrosion and changes in the structure of the specimen such as might be caused by couplings, welds or collars, may be found by eddy-current and flux leakage detectors. Hall effect devices may be used to detect changes in the wall thickness.
Typically, a magnetic probe consists of an elongated magnet that has enlarged pole pieces on each end. The pole pieces are separated from the inner pipe wall by a small clearance. The magnet may be a DC-energized electromagnet or it may be a permanent magnet. The respective sensors may be radially arranged around the magnet core, in close contact with the inner pipe surface or they may be supported on flexibly-mounted shoes that are pressed against the interior wall. As the probe is drawn past a pipe defect, changes in the magnetic flux pattern induce a corresponding transient voltage in the sensors. The transient voltage may be recorded on an oscillograph or other recording medium as a voltage level plotted as a function of linear position of the probe along the specimen.
There are a number of magnetic probes that are available. For example, U.S. Pat. No. 4,789,827, issued Dec. 6, 1988 to Bergander discloses a magnetic flux detection probe that includes first and second flux leakage sensing coils mounted within the probe housing. Sensing coils, radially offset with respect to each other provide a measure of the flux leakage to detect flaws in the pipe under inspection. A Hall effect device senses variations of the pipe thickness.
U.S. Pat. No. 4,088,946 issued May 9, 1978 teaches a magnetic inspection tool that uses a central magnetizer and a Hall effect sensor. Means are provided for nulling out the magnetic field of the main magnetizer so that the Hall effect device will be more sensitive to small flux changes due to pits and corrosion in the piping.
U.S. Pat. No. 4,447,777 issued May 8, 1984 to M. R. G. Sharp et al. teaches a pipeline inspection vehicle which includes a central magnetizer having pole pieces that consist of ferromagnetic wire-brush segments to close the magnetic circuit through the pipe. The brushes rub against the interior pipe wall and also serve as centralizers for the probe. The flux sensor coils are carried on a plurality of shoes, distributed radially around the tool. The sensor shoes are spring-loaded so as to press against the inner pipe wall.
A DC magnetic inspection device for use in cased boreholes is described in a paper presented to the Society of Petroleum Engineers, May, 1991 entitled Full Signature Multiple-Channel Vertilog, by G. W. Adams et al. This device employs a poled electromagnet and associated sense coils for detecting flux leakage and eddy currents caused by pipe defects. The data-gathering system of this tool employs digital technology.
The design of a magnetic probe requires that the cross sectional areas of the pole pieces be larger than the cross sectional area of the tubing. If that is not true, the pipe will be magnetically under saturated. Ideally, the entire magnetic field would be confined to a closed circuit, through the pipe wall, between the pole pieces. In actual fact, there are significant losses in field strength due to open-ended stray fields emanating outwardly from the ends of the magnet. For that reason, the magnetizer element must be relatively large volumetrically to provide a sufficiently strong magnetic field for useful inspection purposes.
In relatively large pipes, exceeding four or five inches in diameter, there is adequate space inside the pipe to accommodate a magnetizer having the required physical size. For small tubing on the order of 2.375" to 3.5" there is simply not enough room for a magnetizer of the necessary cross-sectional area. There is a need therefore for a magnetic flaw detector that can be used in small diameter tubing as well as in tubing of larger diameters.